A vortex valve is a device for controlling fluid flow by a hydraulic effect without requiring moving parts. Such devices have a vortex chamber provided with an outlet at one axial end and an inlet arranged to cause swirl in the chamber when a certain critical flow has been attained. In use, the inlet communicates with a body of water which exerts a pressure head on the liquid entering the vortex chamber. Air is entrained in the liquid drawn through the valve so that, when vortex flow has been established, a central air core exists. U.S. Pat. No. 4,206,783 discloses a vortex valve having a conical vortex chamber with a tangential inlet and an outlet disposed at the narrower end of the chamber. Also known are short vortex valves of which the cross-sectional configuration of the vortex chamber is a logarithmic spiral extending the full length of its longitudinal axis to the outlet. At low flow rates, water entering through the inlet of a vortex valve passes through the vortex chamber to the outlet with substantially no pressure drop and the valve can be considered to be open. However, at high flow rates, water enters through the inlet with enough energy to create a vortex in the vortex chamber which results in a considerable pressure drop between the inlet and the outlet and may greatly restrict flow through the outlet, or even substantially cut it off altogether. Thus the valve serves to limit the rate of flow through it automatically. Vortex valves can be used, for example, to control the flow of storm water in sewers, to ensure that equipment downstream of the valve is not overloaded during periods of heavy rainfall.
DE-A-2658619 discloses a valve for regulating the flow of a liquid or gaseous medium whilst minimizing noise which would be caused by turbulent flow. It is stated that this is achieved by avoiding free boundary surfaces of the flow medium current which emerges from the regulating opening. This contrasts with a vortex valve in which a key element is the establishment of a free boundary surface between the flow medium and the air core, which is an essential feature of a vortex flow control. This feature would be expected to generate noise.
The flow characteristics of a vortex valve flow control (once a vortex has been initiated in the vortex chamber thereof) are dependent on a number of factors including the area of the outlet (A) and the head (H) of fluid upstream of the device. A reasonable approximation of the relationship between the flow (Q) through a vortex valve flow control and the area of the outlet (A) and head (H) is given by the equation: EQU Q=C.sub.d.A (2gH)
where C.sub.d is a coefficient of discharge which is dependent upon the type of vortex valve under consideration, and g is the gravity constant.
Before initiation of the vortex, the rate of flow of fluid through the device is directly dependent upon the head (H) and the area (A) of the outlet. In the "pre-initiation" zone (i.e shortly before initiation), the flow actually decreases somewhat for a small increase in head, before increasing again at initiation at a slower rate than before. This gives rise to what is termed a "pre-initiation bulge" during which the characteristics of the vortex valve are such that it permits a higher rate of flow for a given pressure head than one would expect from a direct extrapolation back towards the origin of the curve at high heads after initiation. In some circumstances, it is desirable to reduce or even eliminate the pre-initiation bulge.
The configuration of a vortex valve determines its flow characteristics, namely its coefficient of discharge (C.sub.d), the extent of pre-initiation bulge and the head required to initiate the vortex.
In some circumstances, it is desirable to control high rates of flow under low head conditions, for instance on river controls or irrigation schemes. At present, this is achieved by the use of orifice flow controls and venturi flumes but these both suffer from disadvantages. To meet such requirements in a vortex valve and for a given outlet area, the C.sub.d should be relatively high and it should also be the case that vortex flow in the vortex valve is initiated at relatively low flows.
It would therefore be desirable to provide a vortex valve having a relatively high C.sub.d and requiring a relatively low head to initiate vortex flow in the vortex valve. This has surprisingly been achieved in accordance with the present invention by the use of a vortex valve having a vortex chamber which is an elongate circular cylinder.